Resolve "Load/Save SFG to file"

Save and load sfg to file, preserves positions when loading the sfg into the gui. Also minor fixes to connection etc.

This can't be merged until the __str__ is fixed for some sfgs. Closes #66 (closed)

b_asic/operation.py

@@ -567,7 +567,6 @@ class AbstractOperation(Operation, AbstractGraphComponent):
             latency_offsets["out" + str(i)] = outp.latency_offset
 
         return latency_offsets
-        #return ([inp.latency_offset for inp in self.inputs], [outp.latency_offset for outp in self.outputs])
 
     def set_latency_offsets(self, latency_offsets: Dict[str, int]) -> None:
         for port_str, latency_offset in latency_offsets.items():